1. Field of the Invention
The present invention relates to detecting serial numbers written on a wafer, for example, serial numbers that identify disk drive heads. More particularly, the present invention relates to using optical character recognition augmented by an error correction code to detect serial numbers written on a wafer.
2. Description of the Prior Art
Microelectronic components, such as microchips and heads employed in disk drives, are typically fabricated on a slice of semiconductor material or ceramic substrate. To enable tracing and quality control of the fabricated components, serial numbers are typically written on the wafers to identify a wafer as a whole, a region of a wafer, as well as each individual component fabricated on the wafer. The serial number is typically written using a human readable code, such as alphanumeric characters, to facilitate identification with visual inspection, but it is also desirable to read the serial numbers using automated machines employing optical character recognition (OCR).
Conventional OCR systems comprise a scanner for scanning the written alphanumeric characters to generate a raster (bit map) representation for each character. The raster image is converted into a detected character by identifying and correlating features of the raster image with expected features corresponding to possible output characters. A confidence metric is generated for each possible output character, and the character having the highest confidence metric is selected as the detected character. This process is susceptible to errors due to imperfections (noise) in the character writing process, as well as the character scanning process. An OCR system may generate an alarm if the confidence metric for a particular character falls below a predetermined threshold, requiring human intervention to verify the accuracy of the detected character through visual inspection.
U.S. Pat. No. 4,105,997 discloses another technique for detecting an error in the characters output by an OCR system by including redundancy characters with the written data characters. The redundancy characters are generated from an error detection code, such as a Fire code, wherein the data characters and appended redundancy characters form an error detection codeword. The redundancy characters are used to decode the detected codewords output by the OCR system in order to detect errors in the data characters. If an error is detected, the OCR system repeatedly rescans the text and decodes the detected codeword until the redundancy characters indicate the data characters are error free. If the error persists, the automated OCR system is aborted and the text image is processed in some alternative manner, such as a human operator performing the translation through visual inspection.
It is undesirable to require human intervention whenever the confidence of the OCR system falls below a threshold or when redundancy characters indicate an error, particularly when processing hundreds of thousands of components fabricated on wafers. There is, therefore, a need to improve the OCR system used to detect serial numbers written on wafers.